This invention relates to papermakers' felts and especially to needled press felts with a woven base fabric that does not require seaming into which is needled a batt consisting of wool or synthetic fibers.
In the conventional papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
After leaving the forming medium, the somewhat self-supporting paper web is transferred to the press section in the machine and onto a press felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well. It is this press felt that is the subject of the present invention.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated generally cylindrical rolls to remove still further amounts of water therefrom.
The base fabrics for the press felts described above can be produced either flat woven and joined or they can be woven endless. A flat woven base fabric must be removed from the weaving machine and joined by seaming. Flat-woven base fabrics will have a tendency to have a different thickness in the joining area where it is seamed than in the remainder of the fabric. Joining a flat-woven base fabric is labor intensive and therefore also expensive. Conversely, in an endless woven fabric, the loom edges tend to have a different thickness than the body of the fabric. Forming the base fabric endless requires a long time in the loom which is, of course, expensive. These non-uniformities in thickness have been known to cause vibrations and bouncing of the press rolls on the paper machine as the thicker part of the felt passes through the press section.
In recent years the speed of papermaking operations have increased to the point that greater roll pressures have been necessitated which cause an increase in the tendency of felts to mark the sheets of paper as they pass through the nip and to wear excessively as they pass over the machine rolls, suction boxes, and cleaning equipment. The problems resulting from the increased production requirements vary depending upon the conditions under which the papermaking operation takes place (temperature, humidity, etc.), the number of abrasive surfaces over which the felt travels, the condition of the papermaking machinery and various other factors known to persons skilled in the art.
Ideally, press felts should have at least the following properties. First, they should have a top surface that is fine enough to produce a smooth finish and minimize marking of the sheet of paper being produced. Second, they should be open enough to allow water to drain through without significant impedance. Third, they should be resilient enough to quickly recover from repeated high nip pressures over a long period of time. Fourth, they should be tough and strong enough to provide good stability, wear resistance and felt life.
Felts with a base fabric assembly having two or more layers of woven fabric, one on top of the other, have been introduced by a number of felt makers. Such woven base fabric can be of either a so-called "duplex" or "triex" construction. Both terms are well known in the art. In felts having a base fabric assembly with two or more layers of woven fabric, the top layer can be woven fine to prevent marking from the coarser machine side bottom layer and to provide good retention of the needled fibers. Also the top woven fabric can be made more compressible than the bottom layer. The other layer, the bottom base fabric layer, can be made relatively coarse so that it has a high void volume and a high degree of compaction resistance and wear resistance. The term "void volume" refers to the empty air space in a woven construction under compression which does not collapse when the fabric is compressed. This volume will receive water expressed during the papermaking process so that the water will not run out or go back into the paper. Void volume in a base fabric is maximized when the yarn is more incompressible and has a larger diameter.
Generally, press felts are assembled in the following manner. If the fabrics are not woven endless, the ends are joined by stitching a seam in a conventional manner. The base fabric is then installed on a needle loom, with the fine layer comprising the outside or top loop where multilayer base fabrics are employed. Batt fibers are applied to the top side or paper contacting surface, in sufficient quantity and weight to give good bulk and cushion properties. The fibers are anchored to the base fabric assembly by one or more needling operations. Thus, the surface of the press felt (top) which contacts the paper web is a felt, formed as the batting material fibers are needled to the base fabric. Fibers may also be needled to the bottom of the woven base fabric to ensure good anchoring of the fibers on the top side.
All press felts whether woven endless or joined by seaming after weaving with either single or double layer construction, contribute significantly to the aforementioned vibration problems in the papermaking process because of the varying thicknesses throughout the body of the press felt.
Accordingly, it is an object of the present invention to provide a less expensive method to prepare an endless press felt for a papermaking or similar machine.
It is a further object of the present invention to provide a more efficient method for joining the woven fabric of the base fabric assembly of a press felt for a papermaking or similar machine.
Another object of the present invention is to provide a method to produce a joined press felt for a papermaking or similar machine which distributes impact of the press rolls from the papermaking machine over a greater area of the press felt.
A further object of the present invention is to provide an improved press felt for the press section of a papermaking machine or similar machine.
It is also a further object of the present invention to provide an improved base fabric construction for a press felt in a papermaking machine.
Still another object of the presen invention is to provide an improved method for making a dual base fabric needled press felt for the press section of a papermaking machine.
It is a further object of the present invention to provide a felt with an improved base fabric construction having uniform thickness so as to significantly reduce or eliminate vibration of the press rolls on the papermaking machine.